A great variety of marking inks are known and an ink for permanent marking will generally contain a color pigment, a filler, and some form of adhesive binder. In product applications involving high temperature exposure as occurs during operation of many electric lamps, it is also known to employ various inorganic binders generally in the form of low melting glasses or glass oxides. Inorganic phosphate binders have also been employed previously in connection with electric lamp manufacture for various purposes and a basing cement having an ALPO.sub.4 binder is disclosed in U.S. Pat. No. 3,080,243, issued Mar. 5, 1963, to Vodicka and assigned to the assignee of the present invention.
More recently, it was discovered that Al.sub.2 O.sub.3 could be reacted with phosphoric acid to provide the binder in a marking ink when placed upon the outside surface of the glass envelope of an incandescent lamp during high speed manufacture of the lamp. More particularly, a liquid dispersion of the marking ink containing color pigment and filler along with the aforementioned binder system was applied to form the ALPO.sub.4 binder in situ with residual heating from the heated glass substrate and provide an adherent coating for the manufacturers monogram. Various problems were encountered with these marking inks, both during application of the liquid dispersion and with performance of the heat-cured film. The liquid dispersions were found to be unstable during machine handling and prone to either excessive thickening until becoming unusable, or becoming too fluid through excessive moisture pickup. The heat-cured films were also prone to being easily abraded and moisture sensitive.
With increasing speeds at which electric lamps are now being manufactured, it becomes desirable to find a marking ink which can be applied more reliably and provides a more permanent indicia means for the lamp. It is not readily possible to substitute other vitreous binder systems in the marking ink in order to improve moisture and abrasion resistance because the known materials require higher temperatures and longer curing periods than are available. Variations in thickness of the glass envelopes employed for lamp manufacture can further modify the conditions of forming a permanent and moisture proof film so that it becomes necessary to find a suitable binder which can be cured at as low a temperature, and in as short a time period as possible.